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Groundwater engineering (or hydrogeology) is a branch of engineering which is concerned with groundwater movement and design of wells, pumps, and drains. The main concerns in groundwater engineering include groundwater contamination, conservation of supplies, and water quality.

Wells are constructed for use in developing nations, as well as for use in developed nations in places which are not connected to a city water system. Wells must be designed and maintained to uphold the integrity of the aquifer, and to prevent contaminants from reaching the groundwater. Controversy arises in the use of groundwater when its usage impacts surface water systems, or when human activity threatens the integrity of the local aquifer system.

Summery

Hydrogeology is a huge subject and has a lot of different branches and parts, such as Subsurface Contaminant Transport, Groundwater Modeling, and Vadose Zone Hydrology. Hydrology is science to study and understand the complex water systems of the Earth and help to solve water problems. Hydrogeology search at how water interacts with geological systems to Understand where it is and how it is moving under the ground to create a better protecting for this resource to be used in ways to help the people. Hydrogeologists can give information on groundwater availability under a one of the different kinds of environment, such as deserts have a lot of underwater because of the nature of sand. The best way to benefits from quality of groundwater is to understand the concept of pollution prevention. It protects the resource and avoids the need for treatment of underwater use.

Dependency on groundwater

In the United States, 51% of the drinking water comes from groundwater supplies. Around 99% of the rural population depends on groundwater. In addition, 64% of the total groundwater of the country is used for irrigation, and some of it is used for industrial processes and recharge for lakes and rivers.

Glossary

Typical acquifer

• Groundwater: Water found underground in spaces in soil and rock. Groundwater is stored in aquifers.

Beginning. STOP: The following definition, Aquifer, has already been included in the Hydrogeology article

• Aquifer: A collection of water underneath the surface, large enough to be useful in a spring or a well. Aquifers can be unconfined, where the top of the aquifer is defined by the water table, or confined, where the aquifer exists underneath a confining bed.

End.

• • Water table: type of aquifer in which groundwater has an open, free surface exposed to the athmosphere.
  • Artesian: type of aquifer in which groundwater is kept under pressure by a system of aquitards and aquicludes.
• Aquitard: a formation which yields considerably less but leaks appreciable quantities of water to drains and wells than an aquifer.
• Aquiclude: a formation which neither yields considerable amounts of water nor has appreciable movement of water through it.
• Aquifuge: a formation that does not possess interconnected pores, thus it is neither porous nor permeable. Its characteristics make it impossible to store water or transmit it.
• Confining Bed: An impermeable layer in an aquifer which encloses water in an aquifer. The presence of a confining bed creates a confined aquifer.
• Porosity: Porosity is a measure of how much empty space is present in a material. More porous materials have more empty space; they are less dense.
• Storativity: The volume of water released or stored per unit surface area per unit decline or increase of head in the aquifer.
• Hydraulic head: The level at which water sits in an aquifer. It is a sum of the pressure head, elevation head and velocity head.
  • Hydraulic gradient: "The change in head per unit distance."
• Hydraulic conductivity: refers to how easily can water move through aquifers. It is related to the amount of water flowing through a given cross sectional area over a given period of time.
• Transmissivity: refers to how well an aquifer can transport water through its thickness.
• Cones of Depression: A Cone of depression occurs when a well is dug and water from an aquifer moves into the well. As water converges on the well, the head of the surrounding aquifer drops into a cone like shape.

History and Development

Henry Darcy, whose work set the foundation of quantitative hydrogeology

Henry Darcy: 19th century

Henry Darcy was a French scientist who made advances in flow of fluids through porous materials. He conducted experiments which studied the movement of fluids through sand columns. These experiments led to the determination of Darcy's law, which describes fluid flow through a medium with high levels of porosity. Darcy's work is considered to be the beginning of quantitative hydrogeology. Darcy’s life associated with both majors engineering and scientific discoveries. Darcy’s lived short life of 54 years and it has three distinct periods: (1) the first period published Darcy’s strong technical background in engineering, mathematics and physics. (2) the longest period of engineering service where Darcy carried out major engineering projects, which is design and construct of the town’s water supply in Dijon. (3) the final years of Darcy’s life, Darcy’s health leads to a shift of research and completing the writing of his life’s work.

Oscar Edward Meinzer: 20th century

Oscar Edward Meinzer was an American scientist who is often called the "father of modern groundwater hydrology". He standardized key terms in the field as well as determined principles regarding occurrence, movement, and discharge. He proved that the flow of water obeys Darcy's law. He also proposed the use of geophysical methods and recorders on wells, as well as suggested pumping tests to gather quantitative information on the properties of aquifers. Meinzer also highlighted the importance of studying the geochemistry of water, as well as the impact of high salinity levels in aquifers.

Aquifers in a Geological System

Darcy's law

Main article: Darcy's law

Darcy's law is a constitutive equation, empirically derived by Henry Darcy in 1856, which states that the amount of groundwater discharging through a given portion of aquifer is proportional to the cross-sectional area of flow, the hydraulic gradient, and the hydraulic conductivity. Governing equations Mass conservation: Min – Mout = ∆M/∆t

          Darcy’s law   ∆M-∆h relationship

An aquifer having a cross sectional area A. Top and bottom are confined by aquitards, and one end is connected to a lake Darcy’s law: qx= K ∂h/∂x qy= −K ∂h/∂y In general, h and q are functions of (x, y, z, t). In this case, h and q depends on (x, t) only Min-Mout= Apw[qx(x0)-qx(x1)] =Apw[qx(x0)-{qx(x0)+ ∆x ∂qx/∂x}] =-A∆xpw ∂qx/∂x=-A∆xpw ∂/∂x(-K∂h/∂x) =A∆xpw ∂/∂x (k ∂h/∂x)

Groundwater flow equation

Main article: Groundwater flow equation

Geometry of a partially penetrating well drainage system in an anisotropic layered aquifer

The groundwater flow equation, in its most general form, describes the movement of groundwater in a porous medium (aquifers and aquitards). It is known in mathematics as the diffusion equation, and has many analogs in other fields. Many solutions for groundwater flow problems were borrowed or adapted from existing heat transfer solutions.

It is often derived from a physical basis using Darcy's law and a conservation of mass for a small control volume. The equation is often used to predict flow to wells, which have radial symmetry, so the flow equation is commonly solved in polar or cylindrical coordinates.

The Theis equation is one of the most commonly used and fundamental solutions to the groundwater flow equation; it can be used to predict the transient evolution of head due to the effects of pumping one or a number of pumping wells.

The Thiem equation is a solution to the steady state groundwater flow equation (Laplace's Equation) for flow to a well. Unless there are large sources of water nearby (a river or lake), true steady-state is rarely achieved in reality.

Both above equations are used in aquifer tests (pump tests).

The Hooghoudt equation is a groundwater flow equation applied to subsurface drainage by pipes, tile drains or ditches. An alternative subsurface drainage method is drainage by wells for which groundwater flow equations are also available.

Calculation of groundwater flow

Relative groundwater travel times.

To use the groundwater flow equation to estimate the distribution of hydraulic heads, or the direction and rate of groundwater flow, this partial differential equation (PDE) must be solved. The most common means of analytically solving the diffusion equation in the hydrogeology literature are:

• Laplace, Hankel and Fourier transforms (to reduce the number of dimensions of the PDE),
• similarity transform (also called the Boltzmann transform) is commonly how the Theis solution is derived,
• separation of variables, which is more useful for non-Cartesian coordinates, and
• Green's functions, which is another common method for deriving the Theis solution — from the fundamental solution to the diffusion equation in free space.

No matter which method we use to solve the groundwater flow equation, we need both initial conditions (heads at time (t) = 0) and boundary conditions (representing either the physical boundaries of the domain, or an approximation of the domain beyond that point). Often the initial conditions are supplied to a transient simulation, by a corresponding steady-state simulation (where the time derivative in the groundwater flow equation is set equal to 0).

There are two broad categories of how the (PDE) would be solved; either analytical methods, numerical methods, or something possibly in between. Typically, analytic methods solve the groundwater flow equation under a simplified set of conditions exactly, while numerical methods solve it under more general conditions to an approximation.

Analytic methods

Analytic methods typically use the structure of mathematics to arrive at a simple, elegant solution, but the required derivation for all but the simplest domain geometries can be quite complex (involving non-standard coordinates, conformal mapping, etc.). Analytic solutions typically are also simply an equation that can give a quick answer based on a few basic parameters. The Theis equation is a very simple (yet still very useful) analytic solution to the groundwater flow equation, typically used to analyze the results of an aquifer test or slug test. Use of the Analytical Method is interesting to answer the sustainability problem because it is current processes. They are intuitive, simple, and based on how activists answer problems.

Numerical methods

The topic of numerical methods is quite large, obviously being of use to most fields of engineering and science in general. Numerical methods have been around much longer than computers have (In the 1920s Richardson developed some of the finite difference schemes still in use today, but they were calculated by hand, using paper and pencil, by human "calculators"), but they have become very important through the availability of fast and cheap personal computers. A quick survey of the main numerical methods used in hydrogeology, and some of the most basic principles are shown below and further discussed in the Groundwater model article.

There are two broad categories of numerical methods: gridded or discretized methods and non-gridded or mesh-free methods. In the common finite difference method and finite element method (FEM) the domain is completely gridded ("cut" into a grid or mesh of small elements). The analytic element method (AEM) and the boundary integral equation method (BIEM — sometimes also called BEM, or Boundary Element Method) are only discretized at boundaries or along flow elements (line sinks, area sources, etc.), the majority of the domain is mesh-free.

General properties of gridded methods

Gridded Methods like finite difference and finite element methods solve the groundwater flow equation by breaking the problem area (domain) into many small elements (squares, rectangles, triangles, blocks, tetrahedra, etc.) and solving the flow equation for each element (all material properties are assumed constant or possibly linearly variable within an element), then linking together all the elements using conservation of mass across the boundaries between the elements (similar to the divergence theorem). This results in a system which overall approximates the groundwater flow equation, but exactly matches the boundary conditions (the head or flux is specified in the elements which intersect the boundaries).

Finite differences are a way of representing continuous differential operators using discrete intervals (Δx and Δt), and the finite difference methods are based on these (they are derived from a Taylor series). For example, the first-order time derivative is often approximated using the following forward finite difference, where the subscripts indicate a discrete time location,

${\frac {\partial h}{\partial t}}=h'(t_{i})\approx {\frac {h_{i}-h_{i-1}}{\Delta t}}.$

The forward finite difference approximation is unconditionally stable, but leads to an implicit set of equations (that must be solved using matrix methods, e.g. LU or Cholesky decomposition). The similar backwards difference is only conditionally stable, but it is explicit and can be used to "march" forward in the time direction, solving one grid node at a time (or possibly in parallel, since one node depends only on its immediate neighbors). Rather than the finite difference method, sometimes the Galerkin FEM approximation is used in space (this is different from the type of FEM often used in structural engineering) with finite differences still used in time.

Application of finite difference models

MODFLOW is a well-known example of a general finite difference groundwater flow model. It is developed by the US Geological Survey as a modular and extensible simulation tool for modeling groundwater flow. It is free software developed, documented and distributed by the USGS. Many commercial products have grown up around it, providing graphical user interfaces to its input file based interface, and typically incorporating pre- and post-processing of user data. Many other models have been developed to work with MODFLOW input and output, making linked models which simulate several hydrologic processes possible (flow and transport models, surface water and groundwater models and chemical reaction models), because of the simple, well documented nature of MODFLOW.

Application of finite element models

Finite Element programs are more flexible in design (triangular elements vs. the block elements most finite difference models use) and there are some programs available (SUTRA, a 2D or 3D density-dependent flow model by the USGS; Hydrus, a commercial unsaturated flow model; FEFLOW, a commercial modelling environment for subsurface flow, solute and heat transport processes; OpenGeoSys, a scientific open-source project for thermo-hydro-mechanical-chemical (THMC) processes in porous and fractured media; COMSOL Multiphysics (FEMLAB) a commercial general modelling environment), FEATool Multiphysics, an easy to use Matlab simulation toolbox, and Integrated Water Flow Model (IWFM), but they are still not as popular in with practicing hydrogeologists as MODFLOW is. Finite element models are more popular in university and laboratory environments, where specialized models solve non-standard forms of the flow equation (unsaturated flow, density dependent flow, coupled heat and groundwater flow, etc.)

Application of finite volume models

The finite volume method is a method for representing and evaluating partial differential equations as algebraic equations. Similar to the finite difference method, values are calculated at discrete places on a meshed geometry. "Finite volume" refers to the small volume surrounding each node point on a mesh. In the finite volume method, volume integrals in a partial differential equation that contain a divergence term are converted to surface integrals, using the divergence theorem. These terms are then evaluated as fluxes at the surfaces of each finite volume. Because the flux entering a given volume is identical to that leaving the adjacent volume, these methods are conservative. Another advantage of the finite volume method is that it is easily formulated to allow for unstructured meshes. The method is used in many computational fluid dynamics packages.

PORFLOW software package is a comprehensive mathematical model for simulation of Ground Water Flow and Nuclear Waste Management developed by Analytic & Computational Research, Inc., ACRi.

The FEHM software package is available free from Los Alamos National Laboratory. This versatile porous flow simulator includes capabilities to model multiphase, thermal, stress, and multicomponent reactive chemistry. Current work using this code includes simulation of methane hydrate formation, CO₂ sequestration, oil shale extraction, migration of both nuclear and chemical contaminants, environmental isotope migration in the unsaturated zone, and karst formation.

Other methods

These include mesh-free methods like the Analytic Element Method (AEM) and the Boundary Element Method (BEM), which are closer to analytic solutions, but they do approximate the groundwater flow equation in some way. The BEM and AEM exactly solve the groundwater flow equation (perfect mass balance), while approximating the boundary conditions. These methods are more exact and can be much more elegant solutions (like analytic methods are), but have not seen as widespread use outside academic and research groups yet.

Water Wells

A water well is a mechanism for bringing groundwater to the surface by drilling or digging and bringing it up to the surface with a pump or by hand using buckets or similar devices. The first historical instance of water wells was in the 52nd century BC in modern day Austria. Today, wells are used all over the world, from developing nations to suburbs in the United States.

here are three main types of wells, shallow, deep, and artesian. Shallow wells tap into unconfined aquifers, and are, generally, shallow, less than 50 feet deep. Shallow wells have a small diameter, usually less than 15 centimeters. Deep wells access confined aquifers, and are always drilled by machine. All deep wells bring water to the surface using mechanical pumps. In artesian wells, water flows naturally without the use of a pump or some other mechanical device. This is due the top of the well being located below the water table. Well location and construction are very important to the safety of the well water. the well should be located away from the rainwater direction so it flows. Rainwater can can carry bacteria and chemicals on with it which cause to contaminate the well water.

Water Well Design and Construction

A water well in Kerala, India.

Proper well design and construction are important to maintain the health of the groundwater and the people which will use the well. Factors which must be considered in well design are:

• A reliable aquifer, providing a continuous water supply
• The quality of the accessible groundwater
• How to monitor the well
• Operating costs of the well
• Expected yield of the well
• Any prior drilling into the aquifer

There are five main areas to be considered when planning and constructing a new water well, along with the factors above. They are:

• Aquifer Suitability
• "Well Design Considerations
• Well Drilling Methods
• Well Screen Design and Development
• Well Testing"

Aquifer suitability starts with determining possible locations for the well using "USGS reports, well logs, and cross sections" of the aquifer. This information should be used to determine aquifer properties such as depth, thickness, transmissivity, and well yield. In this stage, the quality of the water in the aquifer should also be determined, and screening should occur to check for contaminants.

After factors such as depth and well yield are determined, the well design and drilling approach must be established. Drilling method is selected based on "soil conditions, well depth, design, and costs." At this stage, cost estimates are prepared, and plans are adjusted to meet budgetary needs.

Important parts of a well include the well seals, casings or liners, drive shoes, well screen assemblies, and a sand or gravel pack (optional). Each of these components ensures that the well only draws from one aquifer, and no leakage occurs at any stage of the process.

There are several methods of drilling which can be used when constructing a water well:

• "Cable tool
• Air rotary
• Mud rotary
• Flooded reverse circulation dual rotary"

Cable tool drilling is inexpensive and can be used for all types of wells, but the alignment must be constantly checked and it has a slow advance rate. It is not an effective drilling technique for consolidated formations, but does provide a small drilling footprint. Air rotary drilling is cost effective and works well for consolidated formations. It has a fast advance rate, but is not adequate for large diameter wells. Mud rotary drilling is especially cost effective for deep wells. It maintains good alignment, but requires a larger footprint. It has a very fast advance rate. Flooded reverse circulation dual rotary drilling is more expensive, but good for large well designs. It is versatile and maintains alignment. It has a fast advance rate.

Well screens ensure that only water makes it to the surface, and sediments remain beneath the Earth's surface. Screens are placed along the shaft of the well to filter out sediment as water is pumped towards the surface. Screen design can be impacted by the nature of the soil, and natural pack designs can be used to maximize efficiency.

After construction of the well, testing must be done to asses productivity, efficiency and yield of the well, as well as determine the impacts of the well on the aquifer. Several different tests should be completed on the well in order to test all relevant qualities of the well.

Issues in Groundwater Engineering

Contamination

Groundwater contamination happens when other fluids seep into the aquifer and mix with existing groundwater. Pesticides, fertilizers, and gasoline are common contaminants of aquifers. Underground storage tanks for chemicals such as gasoline are especially concerning sources of groundwater contamination. As these tanks corrode, they can leak, and their contents can contaminate nearby groundwater. For buildings which are not connected to a wastewater treatment system, septic tanks can be used to dispose of waste at a safe rate. If septic tanks are not built or maintained properly, they can leak bacteria, viruses and other chemicals into the surrounding groundwater. Landfills are another potential source of groundwater contamination. As trash is buried, harmful chemicals can migrate from the garbage and into the surrounding groundwater if the protective base layer is cracked or otherwise damaged. Other chemicals, such as road salts and chemicals used on lawns and farms, can runoff into local reservoirs, and eventually into aquifers. As water goes through the water cycle, contaminants in the atmosphere can contaminate the water. This water can also make its way into groundwater.

Controversy

Fracking

Contamination of groundwater due to fracking has long been debated. Since chemicals commonly used in hydraulic fracturing are not tested by government agencies responsible for determining the effects of fracking on groundwater, laboratories at the United States Environmental Protection Agency, or EPA, have a hard time determining if chemicals used in fracking are present in nearby aquifers. In 2016, the EPA released a report which states that drinking water can be contaminated by fracking. This was a reversal of their previous policies after a $29 million study into the effects of fracking on local drinking water.

California

California sees some of the largest controversies in groundwater usage due to dry conditions in California, high population, and intensive agriculture. Conflicts generally occur over pumping groundwater and shipping it out of the area, use of groundwater by a commercial company, and contamination of groundwater by development projects. In Siskiyou County in northern California, the California Superior Court ruled poor groundwater regulations have allowed pumping to diminish the flows in the Scott River and disturbed the natural habitat of salmon. In Owens Valley in central California, groundwater was pumped for use in fish farms, which resulted in the death of local meadows and other ecosystems. This resulted in a lawsuit and settlement against the fish companies. Development in southern California is threatening local aquifers, contaminating groundwater through construction runoff. For example, a solar project in San Bernardino County would allegedly threaten the ecosystem of bird and wildlife species because of its use of up to 1,077 acre-feet of groundwater, which could impact Harper Lake.

In September 2014, California passed the Sustainable Groundwater Management Act, which requires users to manage groundwater appropriately, as it is connected to surface water systems.

Colorado

Due to its arid climate, the state of Colorado gets most of its water from underground. Because of this, there have been issues regarding groundwater engineering practices. As many as 65,000 people were affected when high levels of PFCs were found in the Widefield Aquifer. Groundwater use in Colorado back to before the 20th century. Nineteen of Colorado’s 63 countries depends mostly on groundwater for supplies and domestic uses. The Colorado Geological Survey has three significant reports on groundwater in the Denver Basin. The first report Geology of Upper Cretaceous, Paleocene and Eocene Strata in the Southwestern Denver Basin, The second report Bedrock Geology, Structure, and Isopach Maps of the Upper Cretaceous to Paleogene Strata between Greeley and Colorado Springs, The third publication Cross Sections of the Freshwater Bearing Strata of the Denver Basin between Greeley and Colorado Springs.

New Trends in Groundwater Engineering

Since the first wells were made thousands of years ago, groundwater systems have been changed by human activity. Fifty years ago, the sustainability of these systems on a larger scale began to come into consideration, becoming one of the main focuses of groundwater engineering. New ideas and research are advancing groundwater engineering into the 21st century, while still considering groundwater conservation.

Topographical Mapping

New advancements have arisen in topographical mapping to improve sustainability. Topographic mapping has been updated to include radar, which can penetrate the ground to help pinpoint areas of concern. In addition, large computations can use gathered data from maps to further the knowledge of groundwater aquifers in recent years. This has made highly complex and individualized water cycle models possible, which has helped to make groundwater sustainability more applicable to specific situations.

The Role of Technology

Technological improvements have advanced topographical mapping, and have also improved the quality of lithosphere, hydrosphere, biosphere, and atmosphere simulations. These simulations are useful on their own; however, when used together, they help to give an even more accurate prediction of the future sustainability of an area, and what changes can be made to ensure stability in the area. This would not be possible without the advancement of technology. As technology continues to progress, the simulations will increase in accuracy and allow for more complex studies and projects in groundwater engineering.

Growing Populations

As populations continue to grow, areas which were using groundwater at a sustainable rate are now beginning to face sustainability issues for the future. Populations of the size currently seen in large cities were not taken into consideration when the long term sustainability of aquifers. These large population sizes are beginning to stress groundwater supply. This has lead to the need for new policies in some urban areas. These are known as proactive land-use management, where cities can move proactively to conserve groundwater.

In Brazil, overpopulation caused municipally provided water to run low. Due to the shortage of water, people began to drill wells within the range normally served by the municipal water system. This was a solution for people in high socioeconomic standing, but left much of the underprivileged population without access to water. Because of this, a new municipal policy was created which drilled wells to assist those who could not afford to drill wells of their own. Because the city is in charge of drilling the new wells, they can better plan for the future sustainability of the groundwater in the region, by carefully placing the wells and taking growing populations into consideration.

Dependency on Groundwater in the United States

In the United States, 51% of the drinking water comes from groundwater supplies. Around 99% of the rural population depends on groundwater. In addition, 64% of the total groundwater of the country is used for irrigation, and some of it is used for industrial processes and recharge for lakes and rivers. In 2010, 22 percent of freshwater used in USA came from groundwater and the other 78 percent came from surface water. Groundwater is important for some states that don't have access to fresh water. most of the fresh groundwater 65 percent is used for irrigation and the 21 percent is used for public purposes drinking mostly.

References

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